In the present day, ion implanters are often constructed to optimize implantation according to a specific set of applications. In current applications, for example, some beamline ion implanters are configured to generate high current ribbon beams in which the beam cross section that intercepts a substrate has a beam width that is much greater than the beam height. In some configurations the beam width is slightly larger than the size of a substrate at the substrate plane e.g., 200, 300, or 400 mm, while the beam height is on the order of 10 mm, 20 mm, or 30 mm, for example. By scanning the substrate with respect to the ribbon beam in the direction of the beam height, the entire substrate may be implanted by the ion beam.
For other ion implantation applications, it may be preferable to use a spot beam ion beam in which the beam height and beam width are more equal. One advantage afforded by spot beam ion implantation is the better control of dose uniformity afforded by spot beams. In a spot beam ion implantation application, the spot beam may be scanned along a first direction to cover the dimension of a substrate in that direction that is being implanted. At the same time, the substrate may be scanned in a direction perpendicular to that of the scan direction of the spot beam. The local ion dose concentration can be modified by adjusting the speed of the ion beam along the direction of spot beam scanning. This can be accomplished under computer control in a manner that allows the spot beam scanning to be carefully controlled to optimize ion dose uniformity.
In many beamline ion implanters, after exiting a mass resolving slit, the ion beam may propagate as a wide beam of diverging ions to a collimator, which form a collimated ion beam that is directed to the substrate being processed. In order to provide the correct collimation of the ion beam, the collimator is often set to collimate ions that originate from an object that is placed at the plane of the mass resolving slit (MRS). This feature makes it more difficult to operate the same beamline in both spot beam mode and ribbon mode. In ribbon beam mode, the ion trajectories generated by an analyzer magnet may focus at the MRS to fan out into the collimator situated downstream. However in a conventional ion implanter in a spot beam mode the ion beam may pass through the mass resolving slit as a small beam having more parallel ion trajectories. After exiting the mass resolving slit, the spot beam is then deflected back and forth in a scanner by a deflecting field oriented generally perpendicularly to the direction of propagation of the spot beam. This scanning of the spot beam forms a diverging fan of ion trajectories over time that enters the collimator. The object location in this spot beam configuration is within the scanner that is located downstream of the mass resolving slit. The object location of a spot beam generated from a scanner may therefore vary too much from the object location of a ribbon beam for a collimator to properly collimate both types of beams without extensive reconfiguration. Accordingly, it is common practice for a ribbon beam ion implanter to be employed for certain ion implantation steps or for certain substrates, such as high dose implantation, while a separate spot beam ion implanter is employed for other ion implantation steps that require better dose control. It is with respect to these and other considerations that the present improvements have been needed.